In order to be able to comply with requests to operate a PV installation in a curtailed manner, it is necessary to know and track the maximum possible power Pmax of the PV installation at any time, even in the curtailed state of the PV installation, in order to be able to also readjust the power fed into the energy supply grid by the PV installation in the event of a change in the maximum possible power.
Separate radiation sensors are usually used to determine a maximum possible power Pmax of a PV installation. The radiation sensors are PV cells which provide a measurement signal proportional to currently prevailing radiation. If a PV installation is provided with one or more radiation sensors and if the sensors are arranged in such a manner that they representatively capture the radiation of the PV modules of the PV generator, the maximum possible power Pmax of the PV installation can be inferred from the measurement signals from the sensors. However, the radiation sensors are separate components which should be kept available in the PV installation in addition to the PV modules. In addition, the cell types of the radiation sensor and of the PV module generally differ. In addition, the PV cells of the radiation sensor and of the PV module may also be embedded differently. In this case, the embedding of the PV cells characterizes the manner in which the PV cells are installed in their respective housing. These differences cause, in principle, a different electrical and thermal reaction of the PV cells to particular radiation. They therefore require calibration of the sensor measurement signals with respect to the PV modules used, as well as regular calibration checks, which makes this method expensive, in particular for smaller PV installations.
The document US 2011/0196543 A1 discloses a method for determining a maximum possible power of a PV installation operated in a curtailed manner. In this case, the PV installation has at least two installation parts which can be controlled separately. In this case, a first installation part is operated without limiting the maximum possible power. In one embodiment of a method described there, the maximum power of the overall PV installation is estimated with the aid of the power of the first installation part. A second installation part is operated in a curtailed manner in such a way that the PV installation satisfies a predefined curtailment requirement. However, in this case, it is necessary to always operate certain installation parts without restricting their power, which is disadvantageous, at least in the case of overall strong curtailment of the PV installation.
Furthermore, the document DE 10 2010 038 941 A1 discloses a method for determining the maximum possible power of a PV installation with one or more inverters, which is operated in a curtailed manner when averaged over a period. In this case, the inverter(s) of the PV installation is/are operated differently in the period. A maximum possible feed-in amount—and therefore a maximum possible power of the PV installation averaged over the period—is determined from the different operating data relating to the inverter(s). However, a temporal fluctuation in the power fed in by the desired value of the reduced power is tolerated within the period.
The document US 2016/0105146 A1 discloses a method for measuring the maximum power which is available from a photovoltaic array without connected AC devices having to consume power at the time of the measurement. In the method, a power converter connected to the PV array is deactivated for short periods at defined intervals. During these periods, an open-circuit voltage is measured, on the one hand, as soon as it is reached. On the other hand, the PV array is short-circuited via a resistor, in which case a short-circuit current is measured. The measurements are transmitted to a central control circuit for calculating the maximum power.
The article “Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques”, IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 22, JUNE 2007, NO. 2, 439-449, discloses various MPP tracking methods. Whereas an MPP voltage is linked to an open-circuit voltage via a constant in one method, another method discloses the practice of linking an MPP current to a short-circuit current via a further constant.